Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

The following table shows stock-based compensation expense for stock options, RSUs and ESPP (in thousands):



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development	$	1,989	$	1,833	$	3,689	$	3,506
General and administrative	2,174		1,769		3,906		3,557
Total stock-based compensation expense	$	4,163	$	3,602	$	7,595	$	7,063

Valuation Assumptions Using the Black-Scholes Option-Pricing Model

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:



	Six Months Ended June 30,
	2018	2017
Expected dividend yield	0%	0%
Expected volatility	67.8% - 72.2%	66.7% - 67.7%
Risk-free interest rate	2.4% - 3.1%	2.0% - 2.3%
Expected term	6.25 years	6.25 years

Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option and restricted stock unit (RSU) activity during the six months ended June 30, 2018:



	Shares		Weighted- Average Exercise Price			Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2017	4,504,642		$	19.79		7.0
Granted	1,163,517		27.78
Exercised	(185,773	)	5.35
Forfeited or expired	(114,632	)	(23.50		)
Outstanding, June 30, 2018	5,367,754		21.94			7.2	$	18,118
As of June 30, 2018:
Exercisable	3,108,933		18.99			5.9	17,733
Vested and expected to vest	5,141,748		21.75			7.1	18,081